The invention relates to a method of manufacturing an optical fiber having a coating of a protective material such as a synthetic resin. In the method the fiber is drawn from the end of a preform heated to the drawing temperature. The hot fiber is cooled by pulling the fiber through a gas-filled cooling device. The cooled fiber is then passed through a coating device in which the coating material is provided.
The invention also relates to a cooling device for performing the method according to the invention.
A known cooling device is described in, for example, international patent WO 83/02268. The device described in that application consists mainly of a porous pipe through which the fiber is passed. Nitrogen is led through the porous wall of the pipe on all sides into the space enclosed by the pipe. Thermal energy is carried away by the flowing gas. Only the heat-absorbing power of the gas, in this case nitrogen, is used for cooling. Because comparatively large quantities of gas are necessary for this purpose, measures must be taken to prevent the fiber from starting to vibrate under the influence of the gas flow. For this purpose the pipe is porous.
Another known cooling device is described in European Patent Application 0,079,186. In this device, the fiber is cooled only or predominantly by cooled helium which is passed along the fiber. Forced cooling of the fiber permits a higher fiber drawing rate than is possible with natural cooling. When the hot fiber is not cooled by forced cooling and the distance between the preform and the coating device is not sufficiently long, it is possible that at high drawing rates the fiber will not cool by natural processes (radiation etc.) to a temperature which is permissible for providing a protective coating. When the temperature of the fiber, at the moment it is contacted with the protective coating material in the device is too high, the fiber is not sufficiently wetted by the coating material and the coating material may decompose. This may result in a poor quality protective coating.
The device according to European patent application 0,079,186 comprises a tube into which the fiber is guided immediately after drawing. Cooled, dry helium is passed into the tube in such manner that the direction of flow has a component directed radially with respect to the fiber and another component against the direction of movement of the fiber. The tube has a heat-insulating envelope to minimize heat absorption by the cooled helium from the ambient atmosphere. Such heat absorption would reduce the heat-absorbing and hence cooling capacity of the device.
According to a particular embodiment (see also U.S. Pat. No. 4,437,870), the helium is supplied over substantially the whole tube length via a porous tube which surrounds the fiber on all sides. The porous tube in this device is placed within a double-walled tube. The inside diameter of the double-walled tube is larger than the outside diameter of the porous tube. Cooled helium is blown into the space between the porous tube and the inner wall of the double-walled tube. The blown-in helium diffuses through the porous tube into the space inside the porous tube through which the fiber is passed. Since the helium flows from all sides toward the fiber, there is no danger of the fiber starting to vibrate.
Liquid nitrogen is present in the space between the walls of the double-walled tube to prevent the cooled helium from absorbing heat from the surrounding atmosphere. The liquid nitrogen only functions as a heat insulator. In the device, no thermal energy from the hot fiber is dissipated in the liquid nitrogen. Only the heat-absorbing capacity (thermal capacity) of the cooled helium is used to cool the fiber. The thermal energy dissipated by the fiber is drained from the device with the helium. At a drawing rate of 5 m/sec, a helium flow rate (through a device having an inside tube diameter of 12.7 mm) of 39.9 l/min is given in the patent application. At this high gas rate, it is obvious that precautions must be taken to prevent the fiber from starting to vibrate in the cooling device. Without such precautions, vibration might cause a fracture and a asymmetrical synthetic resin coating. In view of the high gas flow rates required in the device according to EP 0,079,186 it may be derived that heat transfer via the gas to the wall of the device and via the wall to the atmosphere is out of the question.